This invention relates to a process for preparing a printed circuit board, more specifically to a process for preparing a printed circuit board, in which a circuit pattern is formed by electroless plating by using a photoresist.
In the prior art, a printed circuit board has been prepared mainly by a subtractive process in which a through-hole is formed by using electroless thin copper plating and electrolytic copper plating in combination and a circuit (wiring) pattern portion is formed by etching. On the other hand, an additive process in which a through-hole and a circuit pattern portion are formed by electroless copper plating has been put to practical use. The additive process is suitable for a fine circuit and a through-hole with a small diameter so that attention has been paid thereto as a process for preparing a high-density printed circuit board to be used in the future.
In the additive process, a resist for electroless copper plating which can stand in an electroless plating solution having high alkalinity (generally pH 11 to 13.5) and high temperature (generally 60.degree. to 80.degree. C.) for a long period of time (generally 4 to 50 hours) is required, and it is difficult to form a fine circuit with each of a line width and a distance between lines being 150 .mu.m by a resist for screen printing so that a photoresist is generally required.
Photoresists for the additive process have been proposed in Japanese Provisional Patent Publications No. 43468/1975, No. 770/1979, No. 100490/1983, No. 199341/1983, No. 12434/1984 and No. 101532/1985. A photoresist having excellent mass productivity by which an electroless copper plating solution is less contaminated has been proposed in Japanese Provisional Patent Publication No. 18692/1988. However, as to all of these proposed photoresists, an organic solvent such as 1,1,1-trichloroethane is used in a developing solution so that there are problems in working environment and a processing cost. Further, use of a halogen-containing organic solvent tends to be limited because of a recent problem of environmental pollution. Thus, there has been demanded a process for preparing a printed circuit board by a photoresist by using a developing solution which ensures good working environment and does not cause a problem of environmental pollution even in the additive process, i.e., a developing solution using no halogen-containing organic solvent.
As a photoresist for electroless plating which can be developed with an alkaline aqueous solution, there has been disclosed an alkaline development type resin composition comprising a combination of a compound obtained by adding hydroxyalkylene (meth)acrylate to a copolymer of maleic anhydride and aromatic hydrocarbon having a vinyl group, and epoxy acrylate having a carboxy group in Japanese Provisional Patent Publication No. 166452/1990.
In Japanese Provisional Patent Publication No. 230154/1990, a photopolymerizable composition comprising a combination of a polymer represented by a compound obtained by adding glycidyl methacrylate to a binary copolymer of styrene and mono-iso-propyl maleate and a polymer represented by a ternary copolymer of benzyl methacrylate, 2-hydroxybutyl methacrylate and methacrylic acid has been disclosed as a resist material for copper plating which can be developed with an alkaline aqueous solution.
However, these proposed photoresists have low tolerance of resistance to electroless copper plating property so that there are problems that pH of a plating solution is heightened and blistering and delamination of the resists are caused when a plating time is long, whereby it is difficult to use the resists in a practical process. Further, in these resists, a large amount of carboxylic acid remains so that the resists absorb water to be blistered in a plating solution having a pH of 12 to 13. There is an essential problem that if plated copper is precipitated in such a state and the resists are shrunk by drying in a drying step, gaps are generated between the resists and the plated copper (see FIG. 1). Existence of these gaps leads to residue of various chemical solutions used in steps subsequent to the plating step and further, a solder resist cannot conform so that electric reliability of a printed circuit board finally obtained is lowered to a great extent.
On the other hand, light-sensitive resin compositions which can be developed with a semi-aqueous solution comprising an alkaline substance and at least one organic solvent have been disclosed in Japanese Patent Publication No. 39895/1972, Japanese Provisional Patent Publications No. 133022/1978, No. 24844/1984, No. 208748/1985 and No. 135936/1988, Japanese Patent Publication No. 28366/1989 and Japanese Provisional Patent Publication No. 220558/1991. However, there has not yet been proposed a process for preparing a printed circuit board in,which a circuit pattern is formed by electroless copper plating by using these light-sensitive resin compositions. As a result of investigation by the present inventors, it has been found that resists prepared by using these light-sensitive resin compositions have problems of poor resistance to electroless copper plating property and contamination of a plating solution.